Foamed polymeric resins have been produced in the past by a continuous method using a suitable screw extrusion apparatus. Pursuant to the prior art method, a cellular product is produced by feeding a particulate polymeric resin into an extruder, melting it by conveying same through a heated section in the first stage of the extruder, and injecting a blowing agent into the molten resin at the beginning of the second stage of the extruder. This is followed by the steps of mixing and cooling the mixture of the molten resin and the blowing agent in the second stage of the extruder where mixing is effected by means of the action of the screw flights and/or other means associated with the screw, and finally extruding the mixture through a die into a zone of lower pressure whereupon the blowing agent expands forming the cellular product. Generally, two extruders are used to provide sufficient cooling capacity.
The problem with the prior art practice resides in the fact that the resin, such as chlorinated polyvinyl chloride resin, is heated to about 400.degree. F. in the first stage in order to melt same and then it is cooled to about 300.degree. F. in the second stage for proper foaming. The difference of about 100.degree. F. between first and second stages imposes a heavy burden on the equipment to provide adequate heating followed by a large cooling capacity. Furthermore, another disadvantage of the prior art practice is that absorption of the blowing agent into molten resin is very slow and requires a longer mixing stage.
It is desirable to process polymeric resins at as low a temperature as possible not only to save energy costs and reduce the size of the equipment but also for the reason that such materials require less stabilizer and are more stable due to a lower heat history profile.
Plasticizers can be used not only to render a resin processable but also to impart the desirable effect of lowering processing temperatures whereby the resin can be melted and otherwise worked at a substantially lower temperature. Although plasticizers can be used to gain an advantage, they do have the detrimental effect of reducing certain physical properties of the resins.
Reduction of processing and melting temperature can also be effected in another way--by using the blowing agent as a functional plasticizer. This can be done by mixing the blowing agent with the particulate resin until a sufficient amount thereof is absorbed by the particulate resin. For instance, depending on the degree of chlorination, chlorinated polyvinyl chloride melts at a temperature in excess of about 350.degree. F. However, when solid particulate chlorinated polyvinyl chloride is mixed with a blowing agent, such as a chlorofluoroalkane, it can be melted at about 300.degree. F.
U.S. Pat. No. 3,366,580 to Kraemer et al describes a chlorinated polyvinyl chloride foam that is made by introducing into a pressure vessel chlorinated polyvinyl chloride powder and a chlorofluoroalkane blowing agent. The contents of the vessel are mixed for many hours at an elevated temperature so that, in conjunction with the generated pressure in the vessel, a substantial portion of the blowing agent is maintained in the liquid phase. Upon completion of the absorption step, contents of the vessel are cooled, removed from the vessel and admixed with a nucleating agent before being extruded into a cellular product.
In Col. 3, lines 42 to 50 of U.S. Pat. No. 3,366,580, it is noted that a foam product can be extruded directly from the chlorinated polyvinyl chloride resin without the intermediate step of mixing particulate resin with a blowing agent. Pursuant to this method, a blowing agent is injected into the melted resin as it is advanced through the extruder. It should be noted that this patent does not teach a continuous method for making a cellular product whereby injection of a blowing agent is made into a solid resin.